Sellers ARE responsible to get their manga to their buyers in the condition stated. Else, what's the point in listing your manga as "LIKE NEW" or "EXCELLENT" condition if you pack so crappy that it ends up with 3 creases, a bent cover, and 2 dents?
Some things to keep in mind:
1)It rains, books are made out of paper, paper getting wet is a bad, bad thing... Luckily, my postman won't, but I've heard others say their postman will leave a package outside. So you should always waterproof your books in some way just in case you're sending them somewhere where it might be raining.
2)Most books are shipped Media Mail, which means anywhere from 3-14 days of bumping around in trucks and being tossed from pile to pile. Sometimes they'll pile boxes on top of each other and if yours is the one on the bottom... >_>
So you should pack your manga with some padding/insulation.
3)Insurance only covers ADEQUATELY packed items. It's true! Each shipping company has guidelines on how certain items need to be packaged for your insurance to be valid and your item covered. Which is why they will often ask for pics of the damage and the packing materials or for you to bring in the package so that they can inspect it when you file a claim.
4) Veteran Ebay sellers say that a package needs to be able to survive a fall down a flight of steps. Are you confident your package and item will survive the fall unharmed? Some will even say it needs to survive a 2 story drop, though I think that's a bit excessive. Surviving a fall from a flight of steps should suffice to protect your package from normal handling.
Use Saran/Plastic Wrap! Almost everyone has some at home and it's easy to use! Wrap the mangas together with the plastic wrap! If you're shipping lots of mangas, you'll probably have 2 piles of books, so just saran wrap each pile separately. (ie, 2 piles of 4 books). This will offer some waterproof protection AND keep the books from jostling around and ending up squished together. Ever received a package where one manga was sandwiched between the covers/pages of another, totally damaging the outer manga?
No saran wrap? Clean white tissue paper (like the ones you wrap presents such as shirts and ties in before you put it in the gift box) works also. You'll need to use about 3 layers though as tissue paper is thin. They go on sale often and are sometimes 75% off after Xmas.
Clean plastic shopping bags are free and they also work great! They'll offer some waterproof protection and when you tape it down, will prevent your mangas from 'dancing' around on their journey to your buyer. Carefully lay your mangas inside, and then tape down the bag securely! I know I've always got some Target bags lying around. XD (idea suggested by indirect)
Kraft paper is good also (though it's more expensive). You can buy them in rolls from any office supply store. I'd wait for a sale or coupon though.
For the love of god, DO NOT use newspaper directly against the manga! The black ink from the newspaper might rub off on the manga edges and you'll have one unhappy buyer. As long as it's not directly against the manga, paper can be good as packing material. You might want to wrap a few layers around the saran/tissue/kraft paper wrapped manga, and then crumple some as cushioning in the box.
Another big NO-NO is rubber-banding the books together. I had one seller rubber-band 2 sets of manga she shipped to me, the tension from the bands left creases on the tops and bottoms of the manga.
Bubblewrap or foam wrap is also good. You can buy this at Walgreens, Rite-Aid, Office Depot, etc. Or if you receive packages, save the bubblewrap, etc. As long as it's clean, no one minds recycled packing materials.
Cardboard Media Mailers are also great!
Now, these are pricey, (about $1-2 a pop when you have them shipped to you unless you get thousands at a time XD) so I'm only adding these here cause there are stores that will send out your purchases using these. So just save them and reuse them if they're in good condition! I flip mine inside out, wrap a book in plastic wrap, or a taped down comic sleeve, then foam wrap, and tape the book mailer shut. These are great cause they're cardboard and so offer a lot of protection. They're also not huge, usually just the perfect size for one book, so if you use Paypal shipping, you can just drop them off in one of those Postal blue mail boxes, rather than having to make a trip to the PO and waiting in line.
Stores that use these cardboard mailers to send out your stuff include: Amazon (suggested by xxaemiliusxx), Blockbuster (suggested by animebaby15), and Rightstuf.com.
Also, if you plan to sell quite a bit of items, consider buying in bulk. I actually buy my foam rolls (I prefer it to bubble wrap) two huge rolls at a time, I also buy bubble mailers and poly mailers in the 100s at a time. You do pay a bit more up front, but I tend to save about $15 per 100 rather than buying them singly or in dozen packs. However this is NOT for those with limited space. My 'packing' area actually takes up about 1/4 of the garage since I have so much packing supplies. >_>
Yes, buying packing supplies will add $.20-$.75 to the shipping costs per package, but ask around, I'm sure buyers are willing to pay a little more to receive WELL PACKED PACKAGES rather than damaged manga.
I tend to ship Media Mail. The reason being that it's usually 1/3 the price of shipping Priority Mail. If you're shipping media mail, consider using a box. Media Mail postage is calculated by the pound and a box is more secure than a bubble mailer. Make sure your box is in good condition though. Cause an already crushed box doesn't offer much protection.
If you prefer Priority Mail, make use of the Flat Rate Priority Mail Envelopes and Flat Rate Boxes.
Within the US: You can fit 4 normal-sized mangas (2 stacks of 2, please wrap them with bubble or foam wrap, etc ~ don't just shove them in the envelope >_>) into the Flat Rate PM envelope and it will be $4.05 shipping regardless of weight. If you're shipping larger manga, like the Ice Kunion, or DMP (Digitial Manga Publishing) mangas, you can fit 3. (one stack of 3)
There are 2 sizes of Flat Rate Box, the one that's better for manga is the shoe-box sized one that can fit from 16-18 mangas, depending on the thickness of the mangas. Once again, manga should be padded somehow, don't just shove manga into a box and assume you've done a great job packing. >_> The flat rate box ships anywhere within the US regardless of weight for just $8.10. HIGHLY RECOMMENDED if shipping to or from Alaska or Hawaii as media mail from or to either state can take forever.
Shipping outside the US? Global Priority Mail is available to many countries! (only one I've had not be on the list is Italy). These flat rate envelopes are almost the same size as the domestic ones. They are a tad shorter in height but they can also still fit 4 regular sized mangas or 3 DMP/Ice Kunion sized ones. I tend to use an extra layer of both kraft paper and foam wrap since it is International. (It's a psychological thing, I think, "It's not in the same country... must add more protection!" even though most GPM packages probably arrive before domestic Media Mail ones do.)
How do I pack a single manga?
You don't have to pack it the way I do, these are just tips, but something to always keep in mind is: If you were the buyer, how would you want your manga purchase to be packed? And what condition do you expect the manga you buy to arrive in?
Have any tips of your own? I'd be happy to add them to this post. =D